“Quantum Tango” and Inverse Isotope Effect in Furan–Water Complex Unraveled by Microwave Spectroscopy and Instanton Theory

Quantum tunneling, a phenomenon central to various physical and chemical processes, is intricately tied to the mass of the tunneling particle. Here, microwave spectroscopy reveals coupled tunneling and inverse H/D isotope effect in a hydrogen-bonded furan-water double-well system. Despite the greater mass of the furan moiety compared to water, deuteration of furan significantly decreases the tunneling splittings, suggesting substantial tunneling involvement. More remarkably, the tunneling splitting doubled upon deuteration of the hydrogen-bonded hydrogen, contrary to typical mass effects on tunneling. <i>Ab initio</i> instanton theory calculations reproduced the tunneling splitting and the anomalous isotope effects, revealing a coupled rotation of both H₂O and furan in opposite directions during tunneling, demonstrating that "it takes two to tango". The inverse H/D isotope effect arises from the variation of the hydrogen bond bending vibrational mode orthogonal to the tunneling pathway, which stems from changes in the hydrogen bond strength during tunneling.